\section{Problem Formulation}
\label{sec:problem}

In our work, in order to manage the various heterogenous data
collected by the mobile devices, we need a common structure for
representing information. One possible solution is to use a
RDF(Resource Description Framework) to represent all the data
collected from the mobile, including user's profile, location, time,
photo, music, message, in/out calls, address book and calender
events.

As mentioned above, there exist strong inter-relationships between
different types of data collected by the mobile devices. Figure 2
provides a good example of it and RDF can represent the relationship
between different types of data. We can consider the whole data as a
graph $G = (V,E,D,T)$, $V$ is the vertex set of the graph $G$, and
here in our case, the nodes could be any type of the data collected
from the mobile devices, such as users, photos, calender, etc. $E$
is the edge set of the graph $G$, which is the representation of the
relationship between different nodes,such as the friendship between
users, or the ownership between photos and users, etc. $D$ is the
content information of the nodes, for example, for each photo, the
user could add a simple sentence or multiple tags to describe it and
for each user, there are some background information about him/her,
etc. $T$ is the type information of the nodes, the type information
is needed for some search and query in our case.


To be more precise, we formulate our problem as follows,
\noindent{\textbf{Input:}} The input is the various heterogenous
data collected form mobile devices. All the data can be represented
by a graph $G=(V, E, D, T)$.

\noindent{\textbf{Querying task:}} We develop a device-centric
toolkit Mquery to process four kinds of queries: Key Query, Neighbor
Query, Shortest Path Query, Subgraph Query.
